1. Field of the Invention
The present invention relates to a recording/reproducing apparatus including an optical pickup having an objective lens with a function of forming an optical spot on an information recording surface of optical recording media of different formats, and more particularly, to a recording/reproducing device including an optical pickup having an objective lens which is compatibly used in a plurality of optical disks each having a different format, such as a digital versatile disk (DVD), a CD-Recordable (CD-R), CD-Rewritable (CD-RW), a Compact Disc (CD) and a Laser Disk (LD).
2. Description of the Related Art
A recording medium for recording and reproducing information such as video, audio or data at high density, is a disk, a card or a tape. However, a disk-type recording medium is primarily used. Recently, an optical disk system has been developed in the form of an LD, a CD and a DVD. However, when optical disks having respectively different formats, such as DVD, CD-R, CD, CD-RW and LD, are compatibly used, optical aberration occurs due to the variation of disk thickness and wavelength. Thus, an optical pickup which is compatible with the different formats of the disks as well as removes the above-mentioned optical aberration has been actively studied. In the result of such a study, optical pickups which are compatible to the different formats have been fabricated.
FIGS. 1A and 1B show a part of a conventional optical pickup which is compatible with different formats. FIG. 1A shows a case where light is focused on a thin optical disk and FIG. 1B is a case where light is focused on a thick optical disk. In FIGS. 1A and 1B, a reference numeral 1 denotes a hologram lens. 2 denotes a refractive objective lens, 3a denotes a thin optical disk, and 3b denotes a thick optical disk. Light 4 output from an unshown light source is diffracted by a grating (lattice) pattern 11 of the hologram lens 1, to accordingly generate non-diffracted zero-order light 40 and diffracted first-order light 41, respectively. The non-diffracted zero-order light 40 is focused on an information recording surface of an optical disk 3a by the objective lens 2. The diffracted first-order light 41 is focused on an information recording surface of an optical disk 3b by the objective lens 2. Therefore, the optical pickup shown in FIGS. 1A and 1B uses the non-diffracted zero-order light 40 and the diffracted first-order light 41 to record information on or read the information from the optical disks 3a and 3b of the different thicknesses, respectively.
Another conventional technology is disclosed in Japanese Patent Laid-open Publication No. Heisei 7-302437, published on Nov. 14, 1995. An objective lens of an optical head apparatus disclosed in the above publication has, from the center of the objective lens, an odd-numbered region(s) having a focal point congruent with an information recording surface of a thin optical disk, and an even-numbered region(s) having a focal point congruent with an information recording surface of a thick optical disk. Thus, in the case of the thin optical disk, light transmitted through the odd-numbered region(s) of the objective lens is used to read information from the thin optical disk. Also, in the case of the thick optical disk, light transmitted through the even-numbered region(s) of the objective lens is used to read information from the thick optical disk.
However, since the optical pickup shown in FIGS. 1A and 1B divides the incident light into zero order light and first order light, the efficiency of a light use efficiency is lowered. That is, since the incident light is divided into zero order light and first order light by the hologram lens 1, only the zero-order light or the first-order light is used to record the information on or read the information from the optical disk, and the optical pickup uses only 15% or so of the incident light, to thereby lower the light use efficiency. Also, according to the thickness of the used optical disk, only one of the zero-order light and the first-order light reflected from the corresponding optical disk 3a or 3b contains actually read information. Thus, the light having no information functions as noise in a light detection operation with respect to the light containing information. The above problem can be overcome by processing the hologram lens 1 of the lens device. However, when working the hologram lens 1, an etching process for producing a fine hologram pattern requires a high precision. Thus, the manufacturing cost increases.
In the case of the prior art disclosed in the Japanese Patent Laid-open Publication No. Heisei 7-302437, the light transmitted through only one of the odd-numbered region(s) and the even-numbered region(s) is used. As a result, the light use efficiency is lowered. Also, since the number of the focal points is always two, the light having no information functions as noise during the light detection, which makes it difficult to detect information from the light reflected from the optical disk.
To solve the above problem, it is an object of the present invention to provide an optical pickup which has an excellent signal detection function independent of a disk format thereof.
It is another object of the present invention to provide an objective lens which compatibly is used with at least two substrates having respectively different thicknesses.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The above and other objects of the present invention are achieved by providing a recording/reproducing apparatus including an optical pickup which is compatible with a plurality of optical recording media, the optical pickup including at least one light source, an objective lens having a function of focusing light emitted from the light source into an optimal light spot on an information recording surface of one of the plurality of the optical recording media, and a light detector to detect light transmitted through the objective lens after being reflected from the information recording surface of the optical recording medium on which the light spot is focused. The objective lens has an inner area, an annular lens area and an outer area which are divided by an annular lens area in a ring form centered at a vertex, wherein the inner area, the annular lens area and the outer area have aspherical surface shapes for focusing light transmitted through the inner area and the outer area into a single light spot by which information can be read from the information recording surface of a relatively thin first optical recording medium and scattered light transmitted through the annular lens area located between the inner area and the outer area cannot be focused on the first optical recording medium during reproduction of the first optical recording medium having a thin substrate. The objective lens focuses light transmitted through the inner area and the annular lens area into a single light spot by which information can be read from the information recording surface of a relatively thick second optical recording medium and scattered light transmitted through the outer area cannot be focused on the second optical recording medium having a thick substrate, during reproduction of the second optical recording medium.
The above and other objects may further be achieved by providing a recording/reproducing apparatus including an objective lens which uses at least two substrates having respectively different thicknesses to use light, the objective lens including an inner area, an annular lens area and an outer area which are divided by an annular lens area in a ring form centered at a vertex, wherein the inner area and the outer area have aspherical surface shapes for focusing light transmitted through the inner area and the outer area into a single light spot by which information can be read from the information recording surface of a relatively thin first substrate. The annular lens area has another aspherical surface shape for scattering light transmitted through the annular lens area and is located between the inner area and the outer area so that the transmitted light cannot be focused on the first substrate with the thinner thickness. The objective lens focuses light transmitted through the inner area and the annular lens area into a single light spot by which information can be read from the information recording surface of a relatively thick second substrate and scatters light transmitted through the outer area so that the transmitted light cannot be focused on the second substrate with the thick thickness.